Lymphocyte entry and migration in tissues play a critical role in immune surveillance and execution of effector functions as well as in diseases such as autoimmunity and metastasis of lymphocyte-derived malignancies. To localize within tissues lymphocytes extravasate across the vascular endothelial cell wall and follow environmental cues present in the tissues. During extravasation and migration within tissues, lymphocytes change shape and extend membrane protrusions, such as filopodia. However, little is known about the specific regulators of actin network remodeling necessary to generate these membrane protrusions and their specific role in T cell extravasation and interstitial migration. Our goal is to determine the mechanistic role of linear actin network remodeling in mediating T cell extravasation and interstitial motility. Most work on actin cytoskeletal remodeling has focused on upstream regulators, such as Rho-GTPases, which have pleiotropic effects on lymphocyte functions. Here instead, we focus on downstream effectors of linear actin network remodeling, Formin and Ena/Vasp proteins. These cytoskeletal effectors control the polymerization of linear actin filaments found in filopodia and we expect they will have a more defined function, such as enabling pathfinding , in regulating lymphocyte extravasation and interstitial migration. Lymphocytes undergo a migratory pathfinding process in order to find permissive extravasation sites along vascular walls and to follow environmental cues during interstitial migration. Filopodia can mediate sensing of environmental stimuli and have been suggested to play a role in lymphocyte trans-endothelial migration (TEM). However, the function of linear actin polymerization and of filopodia in promoting lymphocyte extravasation and interstitial motility remains unclear. We hypothesize that pathfinding during lymphocyte extravasation and interstitial motility requires cytoskeletal effectors of linear actin polymerization to generate specialized membrane protrusions, such as filopodia. We propose three aims to address this crucial gap in our understanding of how lymphocyte extravasation and interstitial motility are regulated by Formin and Ena/Vasp proteins: Aim 1: Determine the role of cytoskeletal effectors of linear actin polymerization in regulating pathfinding during TEM. Aim 2: Define the requirement for cytoskeletal effectors of linear actin polymerization in T cell extravasation and tissue infiltration in vivo. Aim 3: Determine the role of cytoskeletal effectors of linear actin polymerization in T cell pathfinding during interstitial motility and tissue localization. We will use complementary in vivo and in vitro approaches, including cutting-edge intra-vital 2-photon microscopy, to understand how Formin and Ena/Vasp cytoskeletal effectors regulate lymphocyte extravasation and interstitial motility. Determining the molecular basis by which the actin cytoskeleton regulates motility and extravasation of lymphocytes can lead to the design of new therapeutic tools to modulate lymphocyte trafficking and immune responses to treat autoimmunity or prevent metastasis of lymphocytic tumors.